The present disclosure relates generally to apparatus and methods for image sensing, and, more particularly, to a multi-bit quanta image sensor (QIS) having a controllable (e.g., adjustably variable) exposure response characteristic.
The single-bit Quanta Image Sensor (QIS) was disclosed in 2005 (as a digital film sensor) as a possible evolution of solid-state image sensors with integrated signal collection and scanned readout (like CMOS image sensors today) where pixels are shrunk to sub-diffraction limit size, the number of pixels increased substantially to gigapixel or more, read noise reduced to allow reliable single photoelectron detection (e.g., read noise of ˜0.3 e− rms or less, sufficient to discriminate from readout noise), and readout rate increased to avoid saturation of the sensor under normal imaging conditions. See E. R. Fossum, What to do with Sub-Diffraction-Limit (SDL) Pixels?—A Proposal for a Gigapixel Digital Film Sensor (DFS), Proc. of the 2005 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, Karuizawa, Japan, June 2005, the contents of which are hereby incorporated herein by reference in their entirety. Image pixels and image frames are then formed from the resultant spatio-temporal x-y-t data cube. The data cube is composed of bit planes or fields, with pixels formed from “cubicles” of bits in a localized region of 2D space and time. Further aspects of QIS technology and its associated temporally and spatially oversampled imaging methodology have been discussed in more detail in subsequent publications. See, e.g., E. R. Fossum, The Quanta Image Sensor (QIS): Concepts and Challenges in Proc. 2011 Opt. Soc. Am. Topical Meeting on Computational Optical Sensing and Imaging, Toronto, Canada Jul. 10-14, 2011; and S. Masoodian, et al., Early Research Progress on Quanta Image Sensors, in Proceedings of the 2013 International Image Sensor Workshop, Snowbird, Utah USA Jun. 12-16, 2013, each of which is hereby incorporated herein by reference in its entirety.
In the single-bit QIS, each photodetector (“jot”) in the array is binary in nature, with a signal corresponding to either no photoelectron, or one (or more) photoelectrons. The response of the sensor, in terms of bit density (local density of jots that have received a photoelectron during the integration period), gives rise to the so-called D-log H S-shaped response curve, well known in film since the late 1800's. More specifically, the single-bit QIS has a non-linearity in the number of “jots” that have a binary signal of “1” as a function of exposure. This non-linearity is determined by Poisson statistics and also yields significant overexposure latitude compared to conventional CMOS image sensor pixel response as a function of exposure. Detailed analysis of the response of the QIS was presented in 2013, in E. R. Fossum, Modeling the performance of single-bit and multi-bit quanta image sensors, IEEE J. Electron Devices Society, vol. 1(9) pp. 166-174 September 2013, which is hereby incorporated herein by reference in its entirety.
The latter publication also presents analysis for a multi-bit QIS, which was disclosed earlier in 2013, and may be understood as being an in-between image sensor, between CIS/DIS and the single-bit QIS. See, E. R. Fossum, Application of Photon Statistics to the Quanta Image Sensor, in Proceedings of the 2013 International Image Sensor Workshop, Snowbird, Utah USA Jun. 12-16, 2013; E. R. Fossum, Modeling the performance of single-bit and multi-bit quanta image sensors, IEEE J. Electron Devices Society, vol. 1(9) pp. 166-174 September 2013; and S. Chen, A. Ceballos, and E. R. Fossum, Digital Integration Sensor, in Proceedings of the 2013 International Image Sensor Workshop, Snowbird, Utah USA Jun. 12-16, 2013, each of which is hereby incorporated by reference herein in its entirety.
In the multi-bit QIS, the output of each multi-bit jot is an analog signal that is subsequently digitized by an ADC. The digital value is in the range of 0 to 2n−1 where the digital value is equal to the number of photoelectrons captured in the jot during the exposure and n is the bit depth of the ADC in the readout electronics. For example consider n=4. A 4b multi-bit QIS would have an output from 0 to 15, representing the number of photoelectrons that were captured. In other words, compared to the strict binary output of the single-bit QIS, in the multi-bit QIS the digital output signal S is equal to the number of photoelectrons up to the full well capacity, where 0≤S≤2n−1, where the bit depth n is relatively low, such as 1<n≤4-6, corresponding to a full well capacity from perhaps, by way of example, 15 to 63 photoelectrons. The multibit QIS has reduced exposure response non-linearity compared to a single bit QIS, and reduced overexposure latitude. For larger n, the linearity is improved and the overexposure latitude is decreased.